It's an issue I (perhaps obviously) feel strongly about. Many in the industry agree the whole impedance thing is a giant mess. So, even IEM use aside, I very much hope the trend continues towards output impedances < 2 ohms so designers are free to design better headphones and end users don't have to end up with "luck of the draw" pairings when connecting compromised sources to compromised headphones.

When posts, such as yours, claim or imply higher output impedances are somehow better I like to see the factual objective evidence supporting their claim or position. Otherwise, in my opinion, those promoting higher output impedances for questionable reasons are just making things worse.

It's an issue I (perhaps obviously) feel strongly about. Many in the industry agree the whole impedance thing is a giant mess. So, even IEM use aside, I very much hope the trend continues towards output impedances < 2 ohms so designers are free to design better headphones and end users don't have to end up with "luck of the draw" pairings when connecting compromised sources to compromised headphones.

When posts, such as yours, claim or imply higher output impedances are somehow better I like to see the factual objective evidence supporting their claim or position. Otherwise, in my opinion, those promoting higher output impedances for questionable reasons are just making things worse.

RocketScientist
I am in 100% agreement with what you are saying about the desirability of low Z output impedances. It's up to the manufacturers to sort out this mess due to that outdated IEC standard. The main problem at the moment is that very few headphone manufacturers even bother to state whether they meet that standard, or give the optimum source impedance for their headphones.
SandyK

It's so easy for the manufacturer to offer very low output impedance headphone amplifiers and provide a plug in ( or solder ) option to add a series resistor in the output line to give you what ever higher impedance you might want.

It's so easy for the manufacturer to offer very low output impedance headphone amplifiers and provide a plug in ( or solder ) option to add a series resistor in the output line to give you what ever higher impedance you might want.

Agreed. What's surprising is it's not done more. For many DIY'ers here it's not a big deal to add some resistance. But the majority of amp/dac buyers don't even own a soldering iron. So it would be good to have the option.

And the "offer very low output impedance" part is also critical. As many designs inherently have, or require, a higher output impedance for a variety of reasons. With such designs you often can't just bypass a series resistor to lower their output impedance and have them work correctly.

RocketScientest, it may just be that people prefer the sound of the +-7db plot in your graphic.

Sure, it measures like ****, but people might like it. Also, if you are talking IEM's, I would assume the coupling from the tranducer to the eardrum is very good on acount of the fact that the two form a sealed enclosure. A lot of material nowadays is badly recorded with over pumped base (some of that rap stuff is insane).

I think the real reason we have this mess is that back when h/phones first started to get popular and headphone sockets were provided by amplifier manufacturers, they just divided the input down froim the speaker output, or just inserted the dreaded 120 Ohm resistor in series with the socket - cheap and seemed to work ok.

I can see the engineering benefit of a low output impedance design (I'm using 2.7 Ohms on the pre-amp I am working on now), but maybe we need to provide a switchable damping resistor - e.g. 2 Ohms, 15 Ohms, 33 Ohms or 68 Ohms?

@Bonsai, I agree some might like it, but many don't--especially with IEMs as often have very "sharp" (narrow) impedance swings which then create narrow peaks or notches in the frequency response with a higher output impedance. If you've ever played with a parametric EQ (with variable Q) you know the effects of such peaks or notches can sound rather unnatural. I also agree a switched (or even internal jumper) option is a good solution. The user Peranders on here has designed a high quality headphone amp and he's included a jumper to have the output impedance be either 10 ohms or 120 ohms. The minimum of 10 ohms in this case is recommended by the chip manufacture for stability of the very high speed output stage:

@ashock, In my DAC listening test, 20-some people downloaded high quality recordings made of the output of the DACs. In one case I used the SuperFi IEMs on the 6 ohm output impedance of the NuForce uDAC-2. These were "blind" files with generic names. The uDAC-2 recording with the SuperFi's was universally disliked. Everyone who voted on it, voted it the worst even when they didn't know what they were listening to. The trial results are here (it was a very informal listening test):

I have also personally tried 120 ohms vs zero ohms in a blind ABX and the results, not surprisingly, are fairly obvious even with regular dynamic headphones. For example, with the Beyer DT770 Pro 80 ohm studio headphones, the bass is more boomy, exaggerated, with less deep bass extension with 120 ohms in series. With the zero ohm source, the Beyer's bass is much more tight, deep, and controlled.

I eventually plan to publish more on this topic including some better run blind listening trials that include others actually listening to headphones (vs recordings of the output).

RocketScientist, I am in the process of building a "Panda" amp, a Class A design from china that was discussed on RockGrotto. Many members who have K701s commented that it was the best sound they had ever heard from the K701's, particularly with an output impedance of around 100-120 ohms. I think someone mentioned before that the 701's were designed around the IEC standard, so this makes sense. I have some K601's that I suspect will get the same benefit from the power that the Panda can provide, and I have wired 100 ohms in series with the output on their recommendation. I am also building an adapter cable with 100 ohms inline that I can use to compare the SQ of some of the other amps I am building, using the K601's to see which way they sound better..

MrSlim, I agree when headphones are designed for 120 ohms (or some other value significantly greater than zero). I know Sennheiser, Grado, Ultimate Ears, Westone, and others design mainly for zero ohms.

The K701's are known for having relatively accurate flat bass performance. If they were designed for 120 ohm outputs, then they'll be overdamped on a zero ohm source and the bass will likely sound thin. Even if they're designed for zero ohms, some might prefer the "warmer" bass you'd get with 120 ohms in series.

With a headphone like the Beyerdynamic DT 770-80's, with their significantly bumped up low frequency response, the added upper bass boost (and loss of control) from a higher output impedance generally makes for worse sound.

So it's really a mess and that won't change unless the industry moves to zero ohms as a defacto standard. In the meantime, resistance options are the best solution--especially for those of us with many different headphones.